Oil exploration and developing oil wells often pose great financial risks because the costs are substantial. To mitigate some of the financial risks, logging has become essential in nearly every phase of exploration as well as drilling, completing and producing the well. Logging techniques provide information on the depth of formations, the presence of oil, the bottom-hole or formation temperature as well as data associated to the success of completion techniques, initial formation/reservoir pressures and various data related to stimulation treatments that are often applied to increase production rates.
Often the key to attaining an acceptable production rate and its associated financial results lies in the well's response to stimulation techniques (in particular hydraulic fracturing). The technique referred to as hydraulic fracturing describes a process in which a fluid (either thin or viscous) is pumped into the targeted formation at a rate in excess of what can be dissipated through the natural permeability of the formation rock. This results in a pressure build up until such pressure exceeds the strength of the formation rock. When this occurs, the formation rock fails and a so-called “fracture” is initiated. With continued pumping, the fracture grows in length, width and height. At a predetermined time in the pumping process, solid particulate is added to the fluid that is being pumped. This particulate is carried down the well, out of the wellbore and deposited in the created fracture. It is the purpose of this specially designed particulate to keep the fracture from “healing” to its initial position (after pumping has ceased). The particulate is said to be propping open the fracture and is therefore designated as “proppant”. The fracture, which is generated by the application of this stimulation technique, creates a conductive path to the wellbore for the hydrocarbon. Critical to the process of optimizing the design of a hydraulic fracturing treatment, is the determination of the created fracture geometry (in particular fracture length).
Currently there are logging techniques that give limited information on fracture height, but virtually no technique that gives any reliable data connected to propped fracture length.
The lack of an accurate assessment of propped fracture length is due to a combination of factors. First and foremost is the fact that propped fractures can extend for hundreds of feet away from the wellbore. Prior to the development of the technique of the present invention, which utilizes penetrating radar waves, there was no proven technology available that could determine this substantial length aspect (with a reasonable degree of accuracy). Secondly, the down-hole conditions (in particular temperature and pressure) encountered by logging equipment limited the electronic equipment that could be used, types of signals that could be generated and the type of data gathered by this type equipment. It is not uncommon for logging equipment to be subjected to temperatures in excess of 200° C. and pressures up to and exceeding 10,000 psi.
Thus, via logging and other technologies such as pressure analysis and production history matching, the potential productivity of a given well can be more accurately evaluated. However, current logging devices do not address all critical data requirements and more sophisticated equipment may not stand up well to the environmental conditions of a borehole. For example, temperatures may exceed 200° C. down-hole, and this type of heat limits the electronic sensors and circuits that can be used in a logging device.
FIG. 1 shows an example of a typical wellbore that is reinforced with a metal casing 100. Perforations 105 are created in the metal casing at pre-determined depths in the wellbore to enable hydrocarbon (oil or gas) to flow into the casing. A fracturing fluid (either with or without propping agents) is pumped at high pressures through the perforations to create a fracture and to transport the proppant to the designed fracture length. This propping agent (also called proppant) prevents the fracture from closing once pumping has ceased. The predominant fracture configuration is in the form of two wedge-like shapes oriented approximately 180 degrees from each other and extending out from the wellbore. Such a configuration would be characterized by dimensions of width “W”, height “H,” and length “L”. The propped fracture provides a highly conductive conduit for the hydrocarbon to travel from the reservoir into the wellbore.
Ideally, fracture location and orientation, and its dimensions width, height, and length would be known values. However, as mentioned previously, there is limited data available on fracture height and virtually no method available to accurately measure an extended propped fracture length. Therefore, there has been a long-felt need in the art/industry for a logging device that can be used to generate this critical element of fracture geometry while being subjected to the elevated values of temperature and pressure (for example about 200° C., or greater, and 10,000 p.s.i.) associated with down-hole wellbore conditions. There is also a need in the art for a system that can be arranged to operate with existing wellbores that have already been perforated and fracture stimulated and newly drilled wells that may be completed according to the present invention to simplify the measurement process or to enhance its ability to describe the propped geometry generated from a fracturing treatment.